1. Field of the Invention
The invention pertains to a device and a method for the material testing of a test object that contains at least fractions of electrically conductive and ferromagnetic material and features at least one engineered surface with at least one electromagnetic ultrasonic transducer assembly (EMUS) that comprises a permanent or electromagnetic assembly with at least two magnetic poles of different magnetic polarity that face the engineered surface, as well as at least one eddy current coil that is arranged in a projection on the engineered surface between the two magnetic poles in indirect or direct relation to the engineered surface.
2. Description of the Prior Art
It is common practice to utilize electromagnetic ultrasonic transducers for nondestructive material testing and for measuring test objects that consist of electrically conductive materials and preferably also have ferromagnetic properties.
One can distinguish between two types of electromagnetic ultrasonic transducers, namely those that make it possible to generate so-called horizontally polarized shear waves that predominantly propagate within the test object parallel to the coupling surface and US-transducers for generating so-called freely propagating ultrasonic waves that predominantly propagate within the test object perpendicular to the coupling surface. In both instances, the stimulation of ultrasonic waves within the test object can be attributed to magnetostrictions and Lorentz forces produced by the presence of a magnetic field that is largely constant over time in superposition with an electromagnetic alternating field caused by an electric alternating current.
A typical setup for stimulating ultrasonic waves in accordance with the so-called EMUS-principle is shown in FIGS. 5 a and b. Conventional EMUS-transducers 3 feature a permanent magnet 1 as well as an eddy current coil 2 that are realized in the form of one unit and therefore can be jointly operated. The eddy current coil 2 is typically realized in the form of a rectangular or flat spiral coil and arranged on the side of one magnetic pole of the permanent magnet 1 such that the coil 2 is perpendicularly permeated by a permanent magnetic field. If the aforementioned EMUS-transducer 3 is placed on an electrically conductive, ferromagnetic test object 4, the permanent magnetic field and a circuital vector field caused by the eddy current coil are superimposed within the test object such that magnetostrictive effects are induced therein due to the superposition of the magnetic field components of the circuital vector field and the permanent magnetic field that permeates the surface of the test object perpendicularly, wherein Lorentz forces are also generated by the eddy currents induced in the test object such that pressure waves which normally occur perpendicular to the test object surface are ultimately generated in the form of radially polarized shear waves that are able to propagate within the test object in the form of ultrasonic waves. According to the state of the art, both wave types, that is, the ultrasonic waves propagating perpendicular to the test object surface and the ultrasonic waves that propagate parallel to the test object surface due to the radially polarized shear waves, are suitable for use in checking for defects such as, for example, the detection of cracks within the test object, as well as in measuring the wall thickness of the test object.
Since the eddy current coils currently in use are highly sensitive to external mechanical influences, it should be attempted to generally protect such coils from mechanical wear. When examining ferromagnetic test objects, this is further complicated, in particular, in that the eddy current coil situated between the permanent magnet and the test object is literally pressed onto the surface of the test object due to the attractive effect of the magnetic forces and thusly subjected to significant frictional wear.
In this context, German Patent 35 11 076 A1 discloses a pig for electromagnetically testing pipeline walls of steel that makes it possible to examine and detect weak spots caused by corrosion on pipeline walls during the course of a nondestructive test. One pig that is described in greater detail in this publication features electromagnets that are uniformly distributed over the circumference and respectively feature two measuring heads that are axially aligned with one another, a yoke that connects the measuring heads and a magnetizing coil on these measuring heads. In this case, the field of each electromagnet extends parallel to the central pipe axis. The ultrasonic measurements are realized by directly arranging an eddy current coil that is acted upon with strong current pulses having very steep edges on at least one of the poles or magnet heads. Round seams are provided at the junctions of two adjacent pipeline sections and subject the electromagnetic transducer to impact stress when the aforementioned pig travels over the seams as part of a continuous inspection, wherein this impact stress is substantially amplified by the magnetic forces acting between the electromagnets and the pipeline wall. The above-described frictional wear as well as the additional impact stress on the electromagnetic ultrasonic transducer, particularly on the eddy current coil, results in short service lives of EMUS-transducers that should be extended.
Although it is possible to reduce the frictional wear by diminishing the magnetic forces of attraction acting between the EMUS-transducer and the test object to be examined, for example, by lowering the magnetic field induction, this measure would simultaneously result in a substantially lower efficiency of the EMUS-transducer, that is, the force density induced within the test object in order to generate the ultrasonic waves is reduced correspondingly such that the detection sensitivity during the reception of scattered or reflected ultrasonic waves is diminished accordingly.
Another electromagnetic ultrasonic transducer disclosed in European Patent 0 781 994 A2 utilizes two separate eddy current coils for the transmission and the reception to and from an electrically conductive, ferromagnetic test object, wherein said eddy current coils are preferably arranged congruently and provided with an intermediate insulating layer. FIG. 17 of the cited publication shows an EMUS-transducer in which a permanent magnetic field is generated by means of a U-shaped permanent magnet, both magnetic poles of which face a test object to be examined. The eddy current coils that respectively serve for generating as well as for receiving ultrasonic waves and are realized in the form of meander-type coils lie between the surface regions of the test object that is spanned by the U-shaped permanent magnet in a bridge-like fashion. Due to the meander-shaped configuration of the strip conductors, the current directions of two strip conductor sections that extend directly adjacent to one another are oriented opposite to one another. This results in eddy current regions that lie directly adjacent to one another within the test object and generate Lorentz forces with diametrically different directions in superposition with a permanent magnetic field near the surface that is directed parallel to the test object, wherein these Lorenz forces may, in turn, produce pressure waves in the form of horizontally polarized shear waves that propagate along the surface of the test object in the form of ultrasonic waves. Such ultrasonic waves that propagate parallel to the test object surface are well suited for the material testing near the surface, but cannot be used for depth or thickness measurements on the test objects.